IOExample_pytest_test_rootiomixertoomuch test output not completely deterministic

The test IOExample_pytest_test_rootiomixertoomuch occasionally fails (e.g. https://lhcb-nightlies.web.cern.ch/nightly/lhcb-master-mr/12681/LHCb/x86_64_v2-el9-gcc13-opt/tests#IOExample_pytest_test_rootiomixertoomuch) with diffs like

 RootIOMixer          INFO Took event from file 5
 CountBanks           INFO evt got 836 banks
 RootIOMixer          INFO Took event from file 5
-HLTControlFlowMgr   ERROR End of input reached earlier than expected
 RootIOMixer          INFO Took event from file 2
 RootIOMixer       SUCCESS Reading Event record 1 within rootIOMixerInput_2inf.dst
+HLTControlFlowMgr   ERROR End of input reached earlier than expected

It would appear the problem is the exact order of the output messages are not completely deterministic, so now and then differs from the reference.

It should be made deterministic.

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